Pfs28 has been shown to induce antibodies that can block parasite development in mosquitoes. However, the protein is poorly immunogenic. In addition, the transmission blocking activity (TBA) induced by the recombinant Pfs28 seemed less potent compared to that induced by a recombinant Pfs25. Efforts has been placed to characterize the differential transmission blocking activities quantitatively. Further analyses indicated the difference in TBA was due to differential reactivity of recombinant Pfs28 or Pfs25 in ELISA assays: the reactivity of Anti-Pfs28 sera is biased toward the recombinant protein. The anti-Pfs28 sera displayed lower titer in IFA than anti-Pfs25 sera.